Add convenience constructor for BoundingBox and MRA

[stigrj]

Accepts [0,L] or [-L,L] arguments for world size, where L must be a positive integer.

[codecov]

Codecov Report

Merging #179 (64391f3) into master (268d238) will decrease coverage by 0.20%.
The diff coverage is 5.00%.

@@            Coverage Diff             @@
##           master     #179      +/-   ##
==========================================
- Coverage   65.60%   65.39%   -0.21%     
==========================================
  Files         172      172              
  Lines       12441    12479      +38     
==========================================
- Hits         8162     8161       -1     
- Misses       4279     4318      +39     

Impacted Files	Coverage Δ
src/trees/BoundingBox.h	100.00% <ø> (ø)
src/trees/MultiResolutionAnalysis.cpp	65.47% <0.00%> (-8.85%)	⬇️
src/trees/MultiResolutionAnalysis.h	87.50% <ø> (-12.50%)	⬇️
src/trees/BoundingBox.cpp	81.21% <6.66%> (-14.87%)	⬇️

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[robertodr]

Isn't explicit only needed for 1-argument CTORs?

[stigrj]

These are all single argument with the defaults, no?

[robertodr]

Yes they are 😅